Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices include a plurality of optical element units including optical elements, such as lenses and mirrors etc., arranged in the light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image of a pattern formed on a mask, reticle or the like onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical exposure units. In particular with mainly refractive systems, such optical exposure units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually include an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element.
Optical element groups including at least mainly refractive optical elements, such as lenses, mostly have a straight common axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, the optical exposure units holding such optical element groups often have an elongated substantially tubular design due to which they are typically also referred to as lens barrels.
Due to the ongoing miniaturization of semiconductor devices it is desirable to enhance resolution of the optical systems used for fabricating those semiconductor devices. This desire for enhanced resolution obviously pushes the desire for an increased numerical aperture (NA) and increased imaging accuracy of the optical system.
One approach to achieve enhanced resolution is to reduce the wavelength of the light used in the exposure process. In the recent years, approaches have been made to use light in the extreme ultraviolet (EUV) range using wavelengths ranging from 5 nm to 20 nm, typically about 13 nm. In this EUV range it is not possible to use common refractive optics any more. This is due to the fact that, in this EUV range, the materials commonly used for refractive optical elements show a degree of absorption that is too high for obtaining high quality exposure results. Thus, in the EUV range, reflective systems including reflective elements such as mirrors or the like are used in the exposure process to transfer the image of the pattern formed on the mask onto the substrate, e.g. the wafer.
The transition to the use of high numerical aperture (e.g. NA>0.4 to 0.5) reflective systems in the EUV range leads to considerable challenges with respect to the design of the optical imaging arrangement.
Typically, in such a high numerical aperture EUV system, the feature size to be transferred to the substrate goes down to 7 nm or even below. Furthermore, if so called double patterning processes are implemented, the desired properties regarding the overlay accuracy become extremely tight, the desired accuracy going down to 1 nm or less for a single imaging apparatus. As a consequence, either the overlay error contributions of all components of the optical imaging apparatus, in particular, including the error contribution of the optical system as well as of the distortion of the substrate, have to be kept to a level below 100 pm (i.e. a sub-100 pm level) over the entire service life of the apparatus, or some wavefront or image distortion correction device operating in realtime (i.e. during exposure) has to be employed.
Various systems relating to active wavefront correction in one or more degrees of freedom (DOF) are known, for example, from U.S. Pat. No. 6,765,712, U.S. Pat. No. 6,967,756, US 2007/258158 A1, US 2006/018045 A1, DE 10 2007 019 570 A1, US 2006/0193065 A1, US 2004/0008433 A1, EP 1 376 192 A2, U.S. Pat. No. 4,655,563, U.S. Pat. No. 5,986,795, U.S. Pat. No. 4,492,431 and U.S. Pat. No. 6,842,277 B2, the entire disclosure of which is incorporated herein by reference.
These known systems may roughly be divided in two categories. The first category relates to systems where the (wavefront correcting) deformation of an optical surface is generated by deformation forces acting substantially perpendicularly to the optical surface. Such a system is known, for example, from U.S. Pat. No. 6,842,277 B2. This system, however, has the disadvantage that the deformation forces do not only deform the optical surface but also generate rigid body motions of the optical element which have to be corrected or accounted for by a suitable control loop.
The second category relates to systems where the (wavefront correcting) deformation of the optical surface is generated by bending moments introduced via deformation forces acting substantially tangentially to the optical surface. Such a system is known, for example, from US 2002/011573 A1 or U.S. Pat. No. 6,765,712. These systems may be designed in a manner avoiding parasitic rigid body motions by generating a (closed loop) force flow exclusively within the optical element, e.g. by actuators acting between two parallel levers protruding from the optical element. They nevertheless have the disadvantage that the actuators (generating the deformation forces) typically involve external energy supply via cables or the like which cause the introduction of undesired parasitic forces and/or moments into the optical element.
Furthermore, with these systems comparatively high effort is involved for removing heat introduced into the optical element via the deformation actuators without introducing such undesired parasitic forces and/or moments into the optical element (e.g. via cooling fluid ducts or the like).
A further problem that arises with the internally acting design known from US 2002/011573 A1 or U.S. Pat. No. 6,765,712 is the fact that the bending moments generated at the level of the optical surface via the two parallel levers are only equal for the planar mirror surface as disclosed. In case of a curved mirror surface, such parallel levers, due to the different length of the respective lever arm, would cause unequal bending moments at the mirror surface, which would result in (typically undesired) uneven deformation of the optical surface.